1. Field of the Invention
The present invention relates to operational amplifiers and, in particular, to a non-inverting gain stage which receives a single input and generates a single-ended output and which provides increased low frequency gain without introducing high frequency instability.
2. Discussion of the Prior Art
A high gain operational amplifier usually consists of the basic stages shown in FIG. 1. The external input to the amplifier is typically a differential voltage i.e., the difference that exists between the individual voltages applied to two input pins V.sub.IN (+) and V.sub.IN (-). The overall output of the amplifier is the voltage that is available at the output V.sub.out. The output voltage is referred to as a single-ended because, although the input is a differential signal, only one output voltage exists.
The input stage provides high voltage gain to the differential signal applied between its inputs while responding with much lower gain to voltages common to the two inputs. As a result, the differential signal is amplified with little effect from extraneous common-mode signals resulting from, for example, noise pick up. Thus, the input stage of an operational amplifier is designed to minimize errors of the overall amplifier by controlling the error sources at this stage and by developing high gain which reduces the impact of errors introduced in subsequent stages. The input state also provides isolation of input and output quiescent voltage levels by means of its common-mode signal characteristics.
The intermediate stages of an operational amplifier develop additional voltage gain and provide current gain from the input stage to the output stage. Frequently, the major portion of the amplifier's total voltage gain is developed in these intermediate stages. Current gain in these stages provides high current to the output stage without heavy loading of the input stage.
Both differential and single input formats may be used for the intermediate stages. Unless a differential output is desired from the overall amplifier, it is common to use single-ended intermediate gain stages. The choice between the two types is based in part upon power supply rejection ratio, balanced input stage loading, level shifting needs and other factors. As in the input stage, common-mode rejection is developed by the high differential gain and low common-mode gain of a differential stage. However, continued addition of differential rather than single-ended stages results in complexity without benefit. Typically, only one high gain differential gain stage is used.
FIG. 1 shows an operational amplifier with one differential input intermediate gain stage and one single input intermediate gain stage. The first stage is a non-inverting stage with gain A1 and includes a local feed-forward compensation capacitor C.sub.F ; the second stage is an inverting stage with gain A2. Note that one of the inputs of the differential stage must be bypassed by signal passage by capacitor C.sub.B. FIG. 1 also shows a capacitor C.sub.c that maintians the frequency stability of the overall amplifier in a conventional manner by providing conversion of the single-ended output current from the input stage to the output voltage of the intermediate gain stages.
The output stage of an operational amplifier provides impedance isolation from loads by presenting high input impedance to the proceeding intermediate stage and low output impedance to the load. To provide isolation without degrading high frequency performance, the output stage is generally designed to have low input capacitance and wide bandwidth.
FIG. 2 shows a simplified bipolar operational amplifier design of the high gain variety which is known in the prior art. The input differential amplifier stage consists of two common-emitter amplifier transistors Q1 and Q2 which convert the differential voltage applied at the two inputs V.sub.IN (+) and V.sub.IN (-) to differential voltages V.sub.Q1 and V.sub.Q2 at the collectors of devices Q1 and Q2, respectively. Voltage V.sub.Q2 is bypassed at high frequencies by capacitor C.sub.B. Voltage V.sub.Q1 is applied to the inputs of a differential intermediate gain stage which consists of devices Q3, Q4, Q5 and Q6 and a current mirror which consists of devices Q7 and Q8. This stage provides additional voltage gain at low frequencies, the higher frequencies being fed forward around the intermediate circuitry by feed-forward capacitor C.sub.F. It is necessary for stability that the drive to this stage, as supplied by the input stage, be essentially single-ended at high frequency; hence the need for bypass capacitor C.sub.B. Capacitor C.sub.B is often large in value and difficult to integrate on a chip. Its action discards 6dB of high frequency voltage gain.
Thus, the prior art has limitations when applied to low voltage designs and the level of circuit complexity is too great for use in cost-effective, multiple-amplifier-on-a-chip products.